Geochemical Comparison of Two Wetlands in the Eastern Dahomey Basin, Southwestern Nigeria

Abstract

The southern end of southwestern Nigeria is bounded by the Recent alluvium of eastern Dahomey basin, within which is a rich network of wetlands. The uniform geology of these wetlands notwithstanding, they are continually impacted by the level of human activities and processes going on around them. This study therefore was designed to assess and compare the heavy metal impact on the wetlands of Ikorodu and Ikeja, located within the freshwater watersheds on the northern part of Lagos lagoon and explore causes of degradation while suggesting the best mode of wetland conservation and restoration. Topographic maps (Lagos SE sheet 279 and Ijebu-ode SW sheet 280), 18 core samples of wetland sediment in cylindrical PVC core barrel of 30cm length and 10cm diameter were obtained and utilized in this study. The determined pH, TDS (mg/L) and EC (μS/cm) were 2.4-8.0, 21-184 and 31.3-274.6; 4.1-7.6,13-56 and 19.4-83.6; while concentrations (mg/kg) of Pb, Zn, Cu, As, and Cd were; 9.0-614.0, 36.0-1205.0, 22.0-241.0, 6.0-16.0 and 0.6- 100.0; 10.0-61.0, 4.0-350.0, 2.0-35.0 and below detection limit (<0.5), for Ikeja and Ikorodu wetlands respectively. Other environmental-quality indices revealed high to extreme contamination of urban wetlands (Ikeja) while periurban wetlands (Ikorodu) had moderate to low contamination. The evaluated metal concentrations of the two wetlands revealed that the Ikeja wetland sediments have been more impacted, despite the fact that the two wetlands are within the same geological formation. These are indications that local activities such as industrialization, urbanization, and agricultural practices within catchments area have significantly contributed to degradation of Ikeja wetland sediments, compared to that of Ikorodu.